1. Field of the Invention
The system of the present invention relates to underbalanced multilateral drilling and completing of oil wells. More particularly, the present invention relates to a system for drilling and completing a series of multilateral wells off of a single wellbore in an underbalanced system, utilizing a two-string technique, without having to kill the principal wellbore so that all of the multilaterals are drilled or completed while the well is alive.
2. General Background of the Invention
In the drilling of oil wells, one of the most critical elements in drilling has always been to maintain the well in a balanced state, so that should the drill bit strike a pocket of hydrocarbons, that the formation pressure does not overcome the hydrostatic pressure in the well, and thus a blow out does not occur. Likewise, in the completing of oil wells, it is critical that during the completion process, the formation pressure does not overcome the hydrostatic pressure in the well and thus a blowout would not occur. In conventional drilling or completing, what has always been done, is during the drilling or completing process, to flow heavy fluids; i.e., muds, into the drill bore or into the oil well bore, during drilling, so that the hydrostatic pressure of the muds within the bore hole is heavier than the pressure from the formation. Therefore, any potential blowout which may occur otherwise is prevented due to the heavy muds which create the higher hydrostatic pressure downward into the formation.
It has been recently found, that when such a hydrostatic head is placed on the formation, often times the heavy muds or fluids flow into the formation, and by doing so, create severe damage of the formation, which is a detriment to the formation and to the productivity of the well itself. Therefore, there has been developed the technique that is called underbalanced drilling, which technique allows for greater production, and does not create formational damage which would impede the production process. Furthermore, it has been shown that productivity is enhanced in multilateral wells combined with the non-formation damaging affects of the underbalanced drilling. These results are accomplished by introducing a lighter fluid such as nitrogen or air into the drill hole, or a combination of same or other type fluids or gases, sufficiently as to create an underbalance so that fluid in the borehole does not move into the formation during drilling and completion. In order to accomplish this, often times the drilling is undertaken through the use of coiled tubing, which is a continuous line of tubing which unreels off of a spool on the rig floor, and the tubing serves as a continuous drill string for the drill bit at the end of the tubing. Another technique of underbalanced drilling is referred to as micro-annulus drilling where a low pressure reservoir is drilled with an aerated fluid in a closed system. In effect, a string of casing is lowered into the wellbore and utilizing a two string drilling technique, there is circulated a lighter fluid down the outer annulus, which lowers the hydrostatic pressure of the fluid inside the column, thus relieving the formation. This allows the fluid to be lighter than the formation pressure which, if it weren't, would cause everything to flow into the wellbore which is detrimental. By utilizing this system, drillers are able to circulate a lighter fluid which can return up either inner or outer annulus, which enables them to circulate with a different fluid down the drill string. In doing so, basically air and nitrogen are being introduced down the system which allows them to circulate two different combination fluids with two different strings. This also allows for well control during tripping of drilling assembly.
However, when not utilizing a two-string system, the well is being drilled as an underbalanced well. In order to do so, one must kill the well so that the drill string may be tripped out of the hole, until sufficient fluid in the bore to bring the flow to neutral so the wells aren't flowing. When this is done, the fluid which maintains the hydrostatic pressure on the well, may create formation damage because what is actually occurring is sufficient heavy fluid is in the well bore which forces the fluids into the formation thus the well is killed.
Therefore, what is currently being accomplished in the art is the attempts to undertake underbalanced drilling and to trip out of the hole without creating formation damage thereby controlling the pressure, yet hold the pressure so that one can trip out of the well with the well not being killed and maintaining a live well.
It is well known in the art that anytime a heavy fluid must be introduced into the borehole, in order to stop flowing of fluids of the borehole, there is damage being done to the reservoir downhole, which is not desirable. In the prior art which is being submitted with applicant's prior art statement, applicant brings attention to the many articles which have been written on underbalanced drilling, and the techniques which companies are introducing in order to attempt to maintain the wells alive while tripping in and out of the hole. For example, a company called Sperry Sun, in attempting underbalanced drilling, will aerate the fluid into the casing string which lowers the hydrostatic pressure of the well then you proceed to the micro-annulus system which is becoming the method of choice in combination with coiled tubing and jointed pipe systems. However, the basic wells which are being done are regular, singular horizontal wells and even with the micro-annulus system, restricted to a single well either horizontal or vertical.
Therefore, at this time in the art of micro-annulus drilling, what is needed is a system for micro-annulus drilling, utilizing the two string technique, which would allow you to go into drilling multiple radial wells off of the single vertical or horizontal well, without having to kill the well when the radial wells are drilled during the process.